1. Field of the Invention
The present invention relates to a data transmission method and a data transmission apparatus performing two-dimensional interleaving operation along a frequency axis and along a time axis on transmission data, and the data is transmitted according to a multi-carrier transmission form. The present invention may be applied not only to a power-line transmission field, but also, for a cable transmission field, ADSL, VDSL, CATV and so forth, and, for a wireless transmission field, wireless LAN of 2.4 GHz, digital broadcasting, and so forth, and, also, for an optical data transmission field, WDM, and so forth.
2. Description of the Related Art
A system for transmitting data through various kinds of transmission paths such as a cable, radio, and so forth, has been used, and improvement for stable data transmission and improvement of transmission rate are demanded. Moreover, various proposals also for a system for utilizing an existing telephone line or an existing power line as a cable data transmission system have been made. Moreover, for example, as there is an existing power distribution system by which electric power is supplied to each transformer through a 6.6 kV high-voltage power line, and the voltage is lowered to 100 V or 200 V by each transformer, and then, the electric power is supplied to consumers, such as each home, through a low-voltage power line, various proposals for a data transmission system utilizing this low-voltage power line as a data transmission path have been made.
In such a data transmission system utilizing a low-voltage power line, on the side of a high-voltage power line, an optical fiber transmission path is laid along the high-voltage power line, then, at a position of a transformer, a modem is used to connect the optical fiber transmission path with the low-voltage power line, and, also, a modem is used to connect between each terminal apparatus and the low-voltage power line. Thus, data transmission is performed by utilizing the low-voltage power line as a data transmission path.
In such a case, generally speaking, the low-voltage power line includes, for example, an inductance component on the order of 1 μH/m and a capacitance component on the order of 75 pF/m. Therefore, in a case of incoming lines of 150 m and 50 m×30 (houses), equivalently, an inductance of 150 μH and a capacitance of 0.1125 μF are connected. Further, as, in many cases, capacitors for noise prevention is connected to various kinds of household-electric-appliances, the impedance of the power line has a relatively large capacitance. Moreover, in many cases, an inverter drive form is applied to various kinds of household-electric-appliances, and, noises generated from the inverters are inserted into the power line.
Therefore, when using such a low-voltage power line as a data transmission path, data is transmitted through a low-path filter equivalently, thereby, a high-frequency component of the signal attenuates remarkably, and, the signal will contain various noises, in many cases. Moreover, in such a case, although there is comparatively little attenuation of a low-frequency component, noise occurring due to an inverter unit etc. may have many low-frequency components. Therefore, low-frequency components of a signal may include various noises. Moreover, as such a low-voltage power line branches to plurality of lines in general, and, also, no proper impedance matching is performed on each termination end, signal reflection may occur and also problems due to multipath phenomenon may occur.
A system for transmitting data in parallel using a multi-carrier transmission form for a provision for problem concerning mulitpath and line equalization is also known. For example, an OFDM (Orthogonal Frequency Divisional Multipleximg) transmission form, and a DMT (Discrete Multitone) transmission form are known. When such a multi-carrier transmission form is applied, as it is possible to assign a carrier avoiding a frequency band having a large noise level, it becomes possible to transmit data through a low-voltage power line even including many noise components mentioned above. Moreover, as a system in which parallel transmission of data is carried out by the multi-carrier transmission form, for example, an ADSL (Asymmetric Digital Subscriber Line) transmission form is known as a transmission form directed to a high-rate data transmission through a telephone line.
This multi-carrier transmission form is such that transmission data is transformed into parallel form, and modulation is performed by using each carrier having different narrow band, and, in general, a quadrature amplitude modulation (QAM) form is used. Thus, since transmission data is transmitted in parallel, it becomes possible to equivalently reduce the data band rate per channel, therefore to transmit the data by the data transmission path of the low-voltage power line even including much noise, etc.
In the above-mentioned DMT transmission form and OFDM transmission form, it is common to use an inverse fast Fourier transform (IFFT) form and a fast Fourier transform (FFT) form. Moreover, while using error correction code, applying an interleaving technology is also known.
The applicant of the present application previously proposed a data transmission apparatus (modem) in which the function of a Hadamard transform needing processing easier as compared with the function of Fourier transform is used, and, also, a unit of performing two-dimensional interleaving operation along a time axis and along a frequency axis is employed. FIG. 1 illustrates this data transmission apparatus. In this figure, a transmission signal SD is input to a unit for code transformation such as scrambling (SCR), series-to-parallel conversion (S/P), (gray code)-to-(natural code) conversion (G/N), and summing processing. Further, a signal point generating unit 102, an ADM (Hadamard) multiplexing unit 103, a time-and-frequency interleaving unit 104, a DMT (Discrete Multitone) processing unit 105 having functions of inverse fast Fourier transform (IFFT) and guard-time adding, a D-A conversion unit (D/A) 106, and a low-path filter (LPF) 107 are provided. Therethrough, the data is sent out through a TX-line (transmission line).
In the same figure, a reception line, RX-line is connected to a band-path filter (BPF) 110. Further, an A-D conversion unit 111, a DMT processing unit 112 having functions of guard-time removal and fast Fourier transform (FFT), an amplitude-and-phase pulling unit 113, a time-and-frequency inverse fast Fourier transform unit 114, an ADM distribution unit 115, a decision unit (DEC) 116, a code transformation unit 117 having functions of differential processing, (natural code)-to-(gray code) conversion (N/G), parallel-to-series conversion (P/S) and descrambling (DISC), and a synchronization processing unit 118 having functions of sub-frame synchronization and master-frame synchronous function, are provided. Therethrough, a reception signal RD is obtained.
The code transformation unit 101 performs scrambling (SCR) operation and series-to-parallel conversion (S/P) operation on the transmission signal SD, and, then, performs (gray-code)-to-(natural code) conversion (G/N) thereon, so as to generate natural code which can be used for calculation, and, then performs summing operation thereon so that the reception part may perform phase identification by differential calculation. Then, signal points equivalent to sample points at Nyquist intervals are produced by the signal-point generating unit 102. Then, Hadamard transform is performed and, thus, the signal is multiplexed by the ADM multiplex part 103. Then, two-dimensional interleaving along the time axis and along the frequency axis is performed by the time-and-frequency interleaving unit 104. Then, quadrature amplitude modulation guard time addition are performed by the DMT processing unit 105. Then, the data is converted into an analog signal by the D-A conversion unit 106, and the low-path filter 107 passes therethrough only a signal having a frequency band between 10 kHz and 450 kHz, for example, and then, it is sent out to the transmitting line. Instead, a wide transmission band between 2 and 30 MHz may be employed.
The signal having the frequency band between 10 and 450 kHz of the signal received by the reception line RX-line is passed through by the band-pass filter 110 of the reception part, the signal is then converted into a digital signal by the A-D conversion unit 111, DMT demodulation and guard time removal are performed by the DMT processing unit 112, and, by the amplitude-and-phase pulling unit 113, synchronization operation is performed under control of the synchronization processing unit 118. The sub-frame synchronization and master-frame synchronous processing is performed by this synchronization processing part 118.
Then, by the time-and-frequency inverse interleaving unit 114, inverse interleaving operation inverse to the two-dimensional interleaving performed by the time-and-frequency interleaving unit 104 of the transmission end is performed. Then, the ADM distribution unit 115 performs operation inverse to the operation performed by ADM multiplexing unit 103 of the transmission end. Then, the decision unit 116 performs data decision operation and thus, restores the transmission data. Then, the code transformation unit 117 performs differential operation, (natural code)-to-(gray code) conversion (N/G), parallel-to-serial conversion (P/S), and descrambling (DSCR) operation. Thus, the reception signal RD is obtained.
As noise components problematically inserted into the data transmission path is of random ones. For example, FIGS. 2A and 2B illustrate noise characteristics, where the vertical axis represents the signal power while the horizontal axis represents the frequency. As shown in FIG. 2A, the noise changes randomly along the time axis and frequency axis. Then, as mentioned above, by performing quadrature amplitude transform such as Hadamard transform (ADM) or Wavelet transform, etc., and two-dimensional interleaving at the transmission end, and performing inverse interleaving and inverse Hadamard transform etc. at the reception end, the noise components are averaged or equalized, and, thus, have an approximately uniform level, as shown in FIG. 2B. Therefore, since equalization of noise is thus attained, stable data transmission becomes possible.
FIG. 3 illustrates another example of a data transmission apparatus which the applicant of the present application proposed in the past. The same reference numerals are given to the same units as those of FIG. 1. In the configuration of FIG. 3, a roll-off filter and modulation unit (ROF MOD) 108 and a demodulation and roll-off filter unit (DEM ROF) 119 are provided. That is, the roll-off filter and modulation unit 108 is provided between the DMT processing unit 105 and D-A conversion unit 106 in the transmission part shown in FIG. 1, and the demodulation and roll-off filter unit 119 is provided between the A-D conversion unit 111 and the DMT processing unit 113 of the reception part thereof.
The roll-off filter and modulation unit 108 of this data transmission apparatus performs wave shaping operation and digital modulation such as quadrature amplitude modulation on the signal having undergone guard time addition by the DMT processing unit 105. Then, the signal is converted into an analog signal by the D-A converting unit 106, and, then, by the low-pass filter 107, the signal is made to have a predetermined frequency band, and is sent out to the transmission line TX-line. By the demodulation and roll-off filter unit 119, the digital demodulation of the digital signal obtained from the A-D conversion part 111 is carried out, wave shaping operation is carried out by the roll-off filter, and, then, it is input to the DMT processing unit 112.
Then, the DMT demodulation and removal of the guard time are performed by the DMT processing unit 112, synchronization pulling is performed by the amplitude-and-phase pulling unit 113, inverse interleaving is performed by the time-and-frequency inverse interleaving unit 114, operation inverse to the operation performed by the ADM multiplexing unit 103 of the transmission end is performed by the ADM distribution unit 115, the decision unit 116 performs data decision/restoration, and, by the code transformation unit 117, differential operation, (natural code)-to-(gray code) conversion (N/G), parallel-to-series conversion (P/S) and descrambling (DSCR) are performed. Thus, the reception signal RD is obtained.
FIG. 4 illustrates another data transmission apparatus employing a noise removal unit proposed by the applicant of the present application in the past. The same reference numerals as those of FIG. 3 are given to the same units. In the configuration shown in FIG. 4, a zero-point insertion unit 109 and a noise removal unit 120 are provided. The zero-point insertion unit 109 inserts one or a plurality of zeros (zero levels) between signal points of the signal output from the DMT processing unit 105. The noise removal unit 120 extracts noise components accompanying the zero points, obtains noise components accompanying the signal points by processing interpolation operation of the thus-obtained noise components of the zero points, and removes the noise components accompanying the signal points.
FIG. 5 illustrates the noise removal unit which includes a transmission signal generating unit 131 including the code conversion unit, time-and-frequency interleaving unit, etc. of the transmission part, a zero point insertion unit 132 equivalent to the zero point insertion unit 109 shown in FIG. 4, a data transmission paths 133 such as a low-voltage power line, a telephone line, a radio circuit, or the like, a reception signal reproduction unit 134 including the DMT processing unit, time-and-frequency inverse interleaving unit, etc. of the reception part, and the noise removal unit 120 shown in FIG. 4, a frequency shift unit 121, a thinning-out unit (DCM) 122, an interpolation unit (IPL) 123, and a frequency inverse shift unit 124, and a subtraction unit 125.
Into the signal S of 192 kB (bands) given from the transmission signal generating unit 131 shown in FIG. 6, (1), one zero point is inserted between signal points by the zero point insertion unit 132, thus the transmission rate becomes equivalent to twice, 384 kB. Further, as shown in FIG. 6, (2), it corresponds to the case where the number of sample points thus becomes twice along the time axis. In the figure, curves shows a state of an example of amplitude-modulated signal.
When transmission of the transmission signal before zero point insertion is made by 192 kHz, it comes to have a twice frequency band, i.e., 384 kHz by the above-mentioned zero point insertion. Since the signal transmitted through the data transmission path 133 has noise added thereto due to various causes mentioned above, the noise N is added to each signal point and also to each zero point, as shown in FIG. 6, (3). With regard to the frequency axis, the signal inputted into the noise removal unit 120 shown in FIG. 4 is such as that shown in ‘noise distribution’, (1) of FIG. 5, for example. The signal components in this case are shown as a frequency band in a range between −192 kHz and +192 kHz assuming that the central frequency is 0 kHz. As described above, the level of the band components A, B, C, and D are such that A>B>C and D as the noise component in a lower frequency range is higher in level.
The noise removal unit 120 has a configuration including the frequency shift unit 121, thinning-out unit 122, interpolation unit 123, frequency inverse shift unit 124, and subtraction unit 125, and performs a (96 kHz) frequency shift by the frequency shift unit 121. Thereby, as shown in FIG. 5, (2) ‘+96 kHz shift’, the output of the frequency shift unit 121 is such that, the band component A in the range between −192 kHz and −96 kHz is shifted to the range between −96 kHz and 0 kHz; the band component B in the range between −96 kHz and 0 kHz is shifted to the range between 0 and +96 kHz; the band component C in the range between 0 kHz and +96 kHz is shifted to the range between +96 kHz and +192 kHz; and, the band component D in the range between +96 kHz and +192 kHz is shifted to, by turning, the range between −192 kHz and −95 kHz.
Then, by the following thinning-out unit 122, the signal point shown as S+N of the reception signal of FIG. 6, (3), along the time axis, for example, is thinned out. Thereby, only the signal components at the zero points remain, as shown in FIG. 5, (3) ‘thinning out (DCM)’, along the frequency axis. Each arrow in this figure shows band component produced by turning. By the next interpolation unit 123, the noise at the signal point S is obtained by processing interpolation using the noise N at the zero points, and the noise N′ at the signal point between the zero points is obtained by the noise N at the zero points, as shown in FIG. 6, (4). Along the frequency axis, the band in the range between −192 kHz and +192 kHz becomes half, into the range between −96 kHz and +96 kHz, as shown in FIG. 5, (4) ‘interpolation’ (IPL).
In this case, since the frequency band of the noise components thus obtained by the interpolation process is different from the low-frequency band having much noise shown in FIG. 5, (1) ‘noise distribution’, the frequency inverse shift unit 124 performs a (96 kHz) frequency shift. Thereby, it is shifted to the frequency band in the range between −192 kHz and 0 kHz as shown in FIG. 5, (5) ‘−96 kHz shift’, and is input to the subtraction unit 125. By this subtraction unit 125, the signal shown in FIG. 5, (5) ‘−96 kHz shift’ of the output of the frequency inverse shift unit 124 is subtracted from the reception signal shown in FIG. 5, (1) ‘noise distribution’. As a result, the low-frequency noise component is removed, as shown by the broken line shown in FIG. 5, (6) ‘noise removal’. As shown in FIG. 6, (5), the output of the subtraction unit 125 is such that the noise has been removed, and only the signal components S remain. Thus, the transmission signal shown in FIG. 5, (1) ‘transmission 192 kB’ can be restored.
FIG. 7 shows one example of the sampled values and spectrums in the thinning-out unit 122 shown in FIG. 5. In the figure, the left parts of (1) through (4) show the sampled values in amplitude along the time axis while the right parts thereof show the spectrums. With regard to (1) ‘sampled values and spectrums for the signal S(n)’, a value A obtained by Z transform performed on the signal S(n) is expressed by the following formula:A=S(z)=ΣS(n)z−nThe thus-obtained spectrum is in the range between 0 and fs/2, where fs represents the sampling frequency.
With regard to (2) ‘sampled values and spectrums for the signal (−1)n*S(n)’, that is, the a value B obtained by Z transform of the inverted signal of the signal S(n) is expressed by the following formula:B=Z[(−1)nS(n)]=S(−z)In this case, only the signal components in the signal points are inverted, and, the spectrum obtained therefrom becomes the inverted one. When this inverted signal and the signal before the inversion are added together, (3) ‘sampled value and spectrum of the signal t(n)’ are obtained.A value C obtained from Z transform of the signal obtained through this addition is expressed by the following formula:C=Z[t(n)]=T(z)=(½)*S[(z)+S(−z)]where, as the signal t(n) is such that each of t(1), t(3), t(5), . . . is 0, it can be expressed as:T(z)=Σt(2n)*Z−2n.A signal D obtained from thinning out each signal point at t(n)=0 can be expressed as:D=u(n)=T(z1/2)and the final signal E is expressed by the following formula:E=u(z)=[S(z1/2)+S(−z1/2)]/2Thus, it becomes one shown in (4) ‘sampled values and spectrum of the signal u(n)’. Thus, the frequency band is halved and, the result is inputted into the interpolation unit 123 (see FIG. 5).
FIG. 8 shows interpolation processing in the interpolation unit 123. (1) ‘sampled values and spectrum of the signal u(n) corresponds to (4) ’sampled values and spectrum of the signal u(n) shown in FIG. 7. The signal u(n) from the thinning-out unit 122 only includes the noise components. When the zero points are inserted thereto, (2) ‘the sampled values and spectrum of the signal t(n)’ shown in the figure are obtained. A value F obtained from Z transform of this interpolated signal t(n) is expressed by the following formula:F=T(z)=Σt(n)z−nAs each of t(1), t(3), t(5), . . . , is 0,F=Σt(2n)z−n=u(n)z−2nAccordingly,T(z)=U(z2)and, thus, the spectrum comes to have a frequency band in the range between O and fs/2 by turning of the range between 0 and fs/4, as shown in the figure.
As this signal T(z) has the same rate as the signal S(n) and only includes the noise components, the noise components are removable from the reception signal by generating the signal therefrom only having the frequency band of the reception signal by the frequency inverse shift unit 124 (see FIG. 5), and carrying out the subtraction processing by the subtraction unit 125.
Furthermore, an example of a timing generating unit in the synchronization processing unit 118 (see FIG. 4) is shown in FIG. 9. As shown in this figure, a timing extraction unit 140, a phase synchronization unit 150, a power calculation unit (PWR) 141, a band-pass filter (BPF) 142, a vector-generation unit 143, a comparison unit 151, a D-A conversion unit (D/A) 154, a voltage control crystal oscillator (VCXO) 155, a low-path filter (LPF) 152, a frequency dividing unit 156, and a secondary phase synchronization circuit (secondary PLL) 153 are provided.
The vector signal inputted from the roll-off filter (ROF) (the roll-off filter of the demodulation and roll-off filter unit 119 shown in FIG. 4) is processed according to square operation by the power calculation unit 141, and, thus, the power thereof is calculated. Since the above-mentioned zero-point insertion is such that the zero points are inserted at fixed intervals, the frequency components thereof are extracted by the band-path filter 142. For example, the extraction is made where 192 kHz is regarded as the central frequency. Then, by the vector generation unit 143, a vector is generated, that is, combination with signal in quadrature phase is performed, and, then, the result is input to the phase synchronization unit 150.
The comparison unit 151 carries out phase comparison of the output signal of the timing extraction unit 140 with the frequency dividing output signal of the frequency divider 156, and inputs the difference therebetween into the secondary phase synchronization circuit 153 through the low-path filter 152. Two integrators may be included in this secondary phase synchronization circuit 153. The output signal of this secondary phase synchronization circuit 153 is converted into a control voltage of an analog signal by the D-A conversion unit 154, and, thereby, the oscillation frequency of the voltage-controlled crystal oscillator 155 is controlled. The output signal of this voltage-controlled crystal oscillator 155 is input to the A-D conversion unit 117 (see FIG. 4) as a sampling clock signal, and, also, this signal undergoes frequency division by the frequency divider 156, and is regarded as the zero point signal. That is, it is input to the noise removal unit 120 (see FIG. 4) as a timing signal of the zero points.
However, in a system for transmitting data by the multi-carrier transmission form, in order to perform link equation in a reception end, at least 16 channels are needed. In order to obtain two-dimensional interleaved output for these 16 channels, 256 channels of data is needed.
For example, FIG. 10 shows only a transmission part of the data transmission apparatus shown in FIG. 4. In the figure, the number of channels is added thereto for easy understanding, and since 16 channels of data after performing the two-dimensional interleaving operation is to be outputted from the time-and-frequency interleaving unit 104, the ADM multiplex unit 103 should process 256 channels of data and output 16 channels of data (on the input end, 1 sample is included in each channel while, on the output end, 16 samples are included in each channel there). As the zero point insertion unit 109 inserts the zero points between the signal points for 16 channels of data given from the DMT processing unit 105, the zero point insertion unit 109 outputs 32 channels of data.
By the time-and-frequency interleaving unit 104, as shown in FIG. 11, two-dimensional interleaving processing of 256 points of data along the frequency axis in the range between CH1 and CH16 and along the time axis in the range between 1 and 16 is carried out, and 16 channels of data is outputted along the time axis, for example. Thus, since 256 points of data is needed in order to obtain the two-dimensional interleaved output of 16 channels, a problem in that PAR (Peak to Average Ratio) increases may occur. Assuming that the number of channels is n, as PAR=3.01+10 log n [dB], PAR=27 [dB] when n=256. Then, in order to solve this problem, to reduce the level may be possible. However, the reception level is also reduced due to this level reduction, reception S/N is remarkably degraded thereby, and, thus, stable data transmission may become impossible.